1. Field of the Invention
This invention relates to a video camera and more particularly to the automatic focusing (hereinafter occasionally abbreviated to AF) system of a video camera.
2. Description of the Related Art
The video apparatuses such as video cameras have conspicuously advanced during recent years. Their sizes have been reduced. They have come to be arranged to more automatically operate and to have more versatile functions. As a result of the advancement, almost all the video cameras are normally provided with an automatic focus adjustment (or focusing) system these days.
Automatic focus adjustment systems are provided in varied kinds. Unlike a still camera which is arranged to be used in taking a still picture, a video camera is required to be continuously focused on a moving object. It is, therefore, essential for the video camera to be capable of retaining its in-focus state for a moving object.
The performance of the automatic focusing system may be evaluated, in this respect, in terms of stability and quick responsivity. The stability means that a focusing lens is not unnecessarily operated. In other words, it means that no faulty action that causes a blur by unnecessarily moving the focusing lens for focusing occurs. The quick responsivity means that the focusing lens is promptly moved to an in-focus position by correctly determining the direction and speed of the focusing action. The automatic focusing system for a motion picture must be arranged to meet these requirements in a well balanced state and to be capable of responding to any change of an image at a speed apposite to the change. To meet these requirements, the system must have information on the current state of focus and also accurate control information on the operating direction and speed of a focusing motor.
It is a general tendency of these days that the video camera is arranged to extract a signal component which varies with the state of focus from an image signal and to adjust focus on the basis of the signal component extracted. This method permits focus adjustment irrespective of the distance at which a photographed object is located. The automatic focusing systems of the kind obtaining the above-stated information from the image signal can be roughly divided into two kinds. One employs an optical path modulating method which detects focus by modulating an optical path. The other uses a trial method.
In the modulating method, the optical path is modulated by periodically vibrating a lens or an image sensor or the like by means of a piezoelectric element or the like. Information on the result of a discrimination made between a near-focus state, a far-focus state and an in-focus state is thus actively obtained. While it is an advantage of this method that information on the current state of focus and on the driving direction of the focusing motor can be accurately and promptly obtained, a disadvantage of it lies in the addition of the piezoelectric element and a driving circuit for it, which necessitates a complex structural arrangement and an increase in cost of the automatic focusing (AF) system.
In the case of the trial method, AF control information on the focusing lens shifting direction and a focused state, etc., is obtained from changes caused in the image signal by driving the focusing motor. That method is called the trial method as the focusing lens is first tentatively moved to a very small extent. Unlike the modulation method, the trial method permits preparation of the AF system at a low cost as it requires no complex arrangement. However, it is a disadvantage of the trial method that it requires a longer operating time than the modulation method. Another disadvantage of the method resides in an increased probability that a temporal change taking place in the image is undistinguishable from a change brought about by the tentative (or trial) focusing action. Therefore, the control information obtained by that method tends to become ambiguous.
In the event of binary control performed by simply comparing the focus control information with a threshold value, a faulty determination would often be made in accordance with the trial method, if restart of focusing is determined after attainment of an in-focus state. In such a case, the focusing motor would restart despite the in-focus state to greatly degrade picture quality by blurring an image from an in-focus state.
Patent applications filed prior to the present invention relative to automatic focusing include, among others, U.S. Pat. No. 4,762,986 and U.S. Pat. No. 4,804,831 and U.S. patent applications Ser. No. 017,183 filed on Feb. 19, 1987, Ser. No. 046,252 filed on May 5, 1987 and Ser. No. 121,624 filed on Nov. 17, 1987.